Drove home last night, no issues. This morning I go to leave and the truck is stuck, can't even budge it in 4l, forward or back. I quickly realize the back axle is locked solid on both sides.

I immediately assume the worst, that the diff failed and some part fell in and jammed it, preventing both sides from turning.

Not the case, pulled the drums to find that somehow 3 of the 4 spring retainers had failed, everything was just chilling in the bottom of the drum. You can see in the photo how it somehow pulled the pin right through the clip. Everything else looks fine, a little dusty if anything but wear wise, no problems.

Going to try and pick up a spring kit tomorrow and put it all back together.

corroded? thats nothing...ive seen a lot worse than that still holding on...been doing drum brakes for 30 years. i think id check to make sure the backing plate is secure. its also possible that your wheel cylinder is seized on one side

Did you back the breaks off first? looks like you forced the drum over the break shoes and pulled the pins through the retainers.

Brake not break!

This seems a more likely event than " The brakes worked fine when you parked it and the clips forced themselves off while it was parked overnight". I think it is fairly impossible for the clips to fall off while parked.